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Chapter 8-10

Local Area Networks(LANs)

Comparison 4e and 5eCh 7, 4e Ch 8, 5e Ch 10, ForouzanCh 8, 4e Ch 14, 5e Ch 13, ForouzanCh 9, 4e Ch 13, 5e Ch 14, ForouzanCh 10, 4e Ch 15, 5e Ch 14, Forouzan

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Classification TerminologyNetwork technologies classified into three

broad categoriesLocal Area Network (LAN)Metropolitan Area Network (MAN)Wide Area Network (WAN)

LAN and WAN most widely deployed

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The Local Area Network (LAN)

Engineering classificationExtremely popular (most networks are LANs)Many LAN technologies exist

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Key Features of a LANHigh throughputRelatively low costLimited to short distanceOften rely on shared media

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Scientific Justification ForLocal Area Networks

A computer is more likely to communicate with computers that are nearby than with computers that are distant

Known as the locality principle

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TopologyMathematical termRoughly interpreted as “geometry for curved

surfaces”

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Network TopologySpecifies general “shape” of a networkHandful of broad categoriesOften applied to LANPrimarily refers to interconnectionsHides details of actual devices

Fully connected mesh topology (for five devices)

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Bus Topology

Shared medium forms main interconnectEach computer has a connection to the

medium

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Ring Topology

No central facilityConnections go directly from one computer

to another

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Star Topology

Central component of network known as hubEach computer has separate connection to

hub

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Example Bus Network: Ethernet

Most popular LANWidely usedIEEE standard 802.3Several generations

Same frame formatDifferent data ratesDifferent wiring schemes

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Shared Medium in a LANShared medium used for all transmissionsOnly one station transmits at any timeStations “take turns” using mediumMedia Access Control (MAC) policy ensures

fairness

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Illustration of Ethernet Transmission

Only one station transmits at any timeSignal propagates across entire cableAll stations receive transmissionCSMA/CD media access scheme

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CSMA/CD ParadigmMultiple Access (MA)

Multiple computers attach to shared mediaEach uses same access algorithm

Carrier Sense (CS)Wait until medium idleBegin to transmit frame

Simultaneous transmission possible

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CSMA/CD Paradigm(continued)

Two simultaneous transmissionsInterfere with one anotherCalled collision

CSMA plus Collision Detection (CD)Listen to medium during transmissionDetect whether another station’s signal interferesBack off from interference and try again

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Backoff After CollisionWhen collision occurs

Wait random time t1, 0 < t1 < dUse CSMA and try again

If second collision occursWait random time t2, 0 < t2 < 2*d

Double range for each successive collisionCalled exponential backoff

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Media Access on a Wireless Net

Limited rangeNot all stations receive all transmissionsCannot use CSMA/CD

Example in diagramMaximum transmission distance is dStations 1 and 3 do not receive each other’s

transmissions

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CSMA/CAUsed on wireless networksBoth sides send small message followed by data

transmission“X is about to send to Y”“Y is about to receive from X”Data from sent from X to Y

Purpose: inform all stations in range of X or Y before transmission

Known as Collision Avoidance (CA)

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Identifying a DestinationAll stations on shared-media LAN receive all

transmissionsTo allow sender to specify destination

Each station assigned unique numberKnown as station’s addressEach frame contains address of intended

recipient

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Assignment of physical address

The stations may get their address in different ways:

StaticConfigurableDynamic

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Ethernet AddressingStandardized by IEEEEach station assigned by unique 48-bit

addresse.g. 00:30:65:52:2E:96 in hexadecimal form

Address assigned when network interface card (NIC) manufactured (In most cases)

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Ethernet Address Recognition

Each frame contains destination addressAll stations receive a transmissionStation discards any frame addresses to

another stationImportant: interface hardware, not software,

checks address

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Possible DestinationsPacket can be sent to:

Single destination (unicast)All stations on network (broadcast)Subset of stations (multicast)

Address used to distinguish

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Advantages of Address Alternatives

UnicastEfficient for interaction between two computers

BroadcastEfficient for transmitting to all computers

MulticastEfficient for transmitting to a subset of

computers

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Broadcast on EthernetAll 1s address specifies broadcast

(FF:FF:FF:FF:FF:FF in hexcode)Sender

Places broadcast address in frameTransmits one copy on shared networkAll stations receive copy

Receiver always accepts frame that contains this address

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MulticastHalf of addresses reserved for multicastNetwork interface card

Always accepts unicast and broadcastCan accept zero or more multicast addresses

SoftwareDetermines multicast address to acceptInforms network interface card

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Promiscuous ModeDesigned for testing / debuggingAllows interface to accept all packetsAvailable on most interface hardware

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Identifying Frame Contents

Integer type field tells recipient the type of data being carried

Two possibilitiesSelf-identifying or explicit type (hardware record

type)Implicit type (application sending data must

handle type)

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Conceptual Frame Format

HeaderContains address and type informationLayout fixed

PayloadContains data being sent

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Illustration Of Ethernet Frame

Sender placesSender’s address in sourceRecipient’s address in destinationType of data in frame typeCyclic redundancy check in CRC

Figure 14.3 Minimum and maximum length

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Example Ethernet Types

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When Network HardwareDoes Not Include Types

Sending and receiving computers must agreeTo only send one type of dataTo put type information in first few octets of

payloadMost systems need type information

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Illustration of TypeInformation Added to Data

In practiceType information small compared to data carriedFormat of type information standardized

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A Standard For Type Information

Defined by IEEEUsed when hardware does not include type fieldCalled LLC / SNAP header

Logical Link ControlSubNetwork Attachment Point

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Demultiplexing On TypeNetwork interface hardware

Receives copy of each transmitted frameExamines address and either discards or acceptsPasses accepted frame to system software

Network device softwareExamines frame typePasses frame to correct software module

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Network AnalyzerDevice used for testing and maintenanceListens in promiscuous modeProduces

Summaries (e.g., % of broadcast frames)Specific items (e.g., frames from a given address)

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Bus Topology

Any user with a Network Analyzer can read all packets!

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Star Topology

Star Topology and Bus Topology are equal fom security point!

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Ethernet WiringThree schemes

Correspond to three generationsAll use same frame format

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Original Ethernet Wiring

Used heavy coaxial cableFormal name 10Base5Called thicknet

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Second Generation Ethernet Wiring

Used thinner coaxial cableFormal name 10Base2Called thinnet

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Modern Ethernet Wiring

Uses a hubFormal name 10Base-TCalled twisted pair Ethernet

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Ethernet Wiring In An Office

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A Note About Ethernet Topology

ApparentlyOriginal Ethernet used bus topologyModern Ethernet uses star topology

In fact, modern Ethernet isPhysical starLogical busCalled star-shaped bus

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Higher Speed EthernetsFast Ethernet

Operates at 100 MbpsFormally 100Base-TTwo wiring standards10/100 Ethernet devices available

Gigabit EthernetOperates at 1000 Mbps (1 Gbps)Slightly more expensive

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Another LAN Using Bus Topology

LocalTalkDeveloped by

Apple Corp.1984

Simple to use

Slow by current standards

(230,4 kbps)

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Illustration Of LocalTalk

Transceiver required per stationTransceiver terminates cable

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Ring Topology

Second most popular LAN topologyBits flow in single directionSeveral technologies exist

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Token PassingUsed with ring topologyGuarantees fair accessToken

Special (reserved) messageSmall (a few bits)

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Token Passing ParadigmStation

Waits for the token to arriveTransmits one packet around ringTransmits token around ring

When no station has data to sendToken circulates continuously

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Token Passing Ring Transmission

Station waits for token before sendingSignal travels around entire ringSender receives its own transmission

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Strengths of Token Ring Approach

Easy detection ofBroken ringHardware failuresInterference

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Weaknesses of Token Ring Approach

Broken wire disables entire ringPoint-to-point wiring

Awkward in office environmentDifficult to add / move stations

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Token Passing Ring Technologies

ProNet-10Operated at 10 Mbps

IBM Token RingOriginally operated at 4 MbpsLater version operated at 16 Mbps

Fiber Distributed Data Interconnect (FDDI)Operated at 100 Mbps

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FDDI TerminologyFDDI

Uses optical fibersHigh reliabilityImmune to interference

CDDIFDDI over copperSame frame formatSame data rateLess noise immunity

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FDDI Hub TechnologyPart of FDDI standardStations attach to hubSame frame format and data rate as FDDICalled star-shaped ring

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FDDI Failure RecoveryUses two ringsAutomatic failure recoveryTerminology

Dual-attachedCounter rotatingSelf healing

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Illustration of FDDIFailure Recovery

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Another Example of aPhysical Star Topology

Asynchronous Transfer Mode (ATM)Designed by telephone companiesIntended to accommodate

VoiceVideoData

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ATM

Building block known as ATM switchEach station connects to switchSwitches can be interconnected

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Details of ATM Connection

Full-duplex connectionsTwo fibers required

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ATM CharacteristicsHigh data rates (e.g. 155 Mbps)Fixed size packets

Called cellsImportant for voice

Cell size is 53 octets48 octets of data5 octets of header

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SummaryLocal Area Networks

Designed for short distanceUse shared mediaMany technologies exist

Topology refers to general shapeBusRingStar

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Summary (continued)

AddressUnique number assigned to stationPut in frame headerRecognized by hardware

Address formsUnicastBroadcastMulticast

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Summary (continued)

Type informationDescribes data in frameSet by senderExamined by receiver

Frame formatHeader contains address and type informationPayload contains data being sent

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Summary (continued)

LAN technologiesEthernet (bus)IBM Token RingFDDI (ring)ATM (star)

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Summary (continued)

Wiring and topologyCan distinguish

Logical topologyPhysical topology (wiring)

Hub allows Star-shaped busStar-shaped ring

Figure 14.1 Three generations of Ethernet

Figure 14.5 Unicast and multicast addresses

Figure 14.6 Physical layer

Figure 14.7 PLS

Figure 14.8 AUI

Figure 14.9 MAU (transceiver)

Figure 14.10 Categories of traditional Ethernet

Figure 14.11 Connection of a station to the medium using 10Base5

Figure 14.12 Connection of stations to the medium using 10Base2

Figure 14.13 Connection of stations to the medium using 10Base-T

Figure 14.14 Connection of stations to the medium using 10Base-FL

Figure 14.15 Sharing bandwidth

Figure 14.16 A network with and without a bridge

Figure 14.17 Collision domains in a nonbridged and bridged network

Figure 14.18 Switched Ethernet

Figure 14.19 Full-duplex switched Ethernet

14.2 Fast Ethernet14.2 Fast Ethernet

MAC Sublayer

Physical Layer

Physical Layer Implementation

Figure 14.20 Fast Ethernet physical layer

Figure 14.21 MII

Figure 14.22 Fast Ethernet implementations

Figure 14.23 100Base-TX implementation

Figure 14.24 Encoding and decoding in 100Base-TX

Figure 14.25 100Base-FX implementation

Figure 14.26 Encoding and decoding in 100Base-FX

Figure 14.27 100Base-T4 implementation

Figure 14.28 Using four wires in 100Base-T4

14.3 Gigabit Ethernet14.3 Gigabit Ethernet

MAC Sublayer

Physical Layer

Physical Layer Implementation

Figure 14.29 Physical layer in Gigabit Ethernet

Figure 14.30 Gigabit Ethernet implementations

Figure 14.31 1000Base-X implementation

Figure 14.32 Encoding in 1000Base-X

Figure 14.33 1000Base-T implementation